The present invention relates to a method of and an apparatus for detecting weft yarn insertion in a jet loom such as a water jet loom or an air jet loom.
Water jet looms or air jet looms, also called shuttleless looms, utilize a jet of water or air to carry the weft yarn through the shed. The jet looms include a weft stop device for automatically stopping operation of the loom in response to detection of a weft insertion failure.
The weft yarn as inserted by a water jet is normally detected by a weft feeler in the form of an electrode, and the weft yarn as carried by an air jet is normally detected by a photoelectric feeler. FIG. 1 of the accompanying drawings illustrates the original waveform of a signal A generated by such a weft yarn detector associated with a water jet loom, the signal A being plotted in one cycle of principal motion of the loom. The illustrated motion cycle is composed of an internal I in which there is no signal generated, an interval II in which only a jet of water is ejected, an interval III in which atomized water and a weft yarn are present, an internal IV in which only the weft yarn is present in the shed, and an interval V in which the inserted weft yarn is beaten up by a reed. The waveform of the signal A or the signal portion in the interval IV is identified to detect whether there is a weft yarn length inserted through the shed. More specifically, the conventional weft yarn detecting apparatus detects weft yarn insertion by calculating the proportion of a portion of the signal A which exceeds a certain threshold in an interval, or comparing an integrated or differentiated value of a signal portion in an interval with a threshold.
Where amplifiers for amplifying such a signal are DC-coupled with each other, however, a signal produced due to deteriorated insulation is also amplified by the amplifiers, resulting in difficulty in achieving correct weft insertion determination. Where the amplifiers are AC-coupled with each other to avoid this difficulty, the signal portion in the interval IV tends to go negative with a weft yarn having a low moisture content. No matter how the amplifiers are coupled, the time when the interval IV starts after the interval III varies at all times. This makes it difficult to effect reliable determination of weft insertion in the interval IV based on the signal waveform under the influence of large signal irregularities in the interval II. The threshold against which the signal A is to be compared is not absolute, but varies relative to the signal. With comparison between a simply processed signal waveform and a threshold, therefore, it has been impossible to detect weft yarns correctly, and weft detection failures and unnecessary loom shutdown have been caused frequently.